The results are summarized as follows;(1) Dietary urea is not degraded at the gastro-intestinal tract, and absorbed in intact form from the jejunum. (2) The absorbed urea is not hydrolyzed but excreted into tubular urine. (3) The urinary urea is carried back from the cloaca into the ceca by antiperistalsis, hydrolyzed there to ammonia by cecal microflora and absorbed from the cecal wall. (4) A part of the absorbed ammonia is utilized for synthesis of non-essential amino acids in the liver and a part is excreted in the form of uric acid by way of glutamine formation. (5) Other urinary nitrogenous compounds are also recovered by the pathway through ceca and metabolized in a similar manner to urea. (6) The recovered nitrogen by this nitrogen recycling system through ceca is utilized effectively in the protein-depleted chicken but less utilized in the protein-adequate chickens. (7) The entry into the ceca of digesta escaped from intestinal absorption adversely exerts on dietary nitrogen retention.(8) Nitrogen retention in chickens is based on balance of two entries of digesta and urine. (9) Ceca beneficially function in nitrogen nutrition of wild birds under severe nutritional conditions but not in well-nourished chickens. (10) Cecal ligation does not affect metabolizable energy value of diets. (11) Cecal microflora is regulated by the entry of digesta and urine, ambient circumstances around hosts and dietary fiber such as polyphenol and oligosaccharide. (12) Dietary pectin increases cecal weight and area.It is concluded that the ceca of chickens play an important role in nitrogen metabolism according to protein supply, but not any role in energy metabolism, and that dietary fiber is involved in nutrient utilization through cecal fermentation.